The present invention relates to a method for forming a thin film pattern, and more particularly, when the thin film pattern is to be formed at a step portion, the present invention relates to a method for forming a thin film pattern with a very fine (or high accuracy) shape at the lower part of the step portion. The present invention also relates to a method for fabricating a magnetic head using the method for forming a thin film pattern.
Thin film techniques have been widely used in the fields of fabricating a semiconductor device, a magnetic bubble memory, a thermal head, a thin film magnetic head, etc. In recent years, with the development of high integration and stacking of the device, it has been required to form a thin film pattern with a very fine shape at a step portion.
For example, with the development of high density recording in a magnetic disk system for use in a large scale computer, the recording track of a thin film magnetic head for write or read for a disk is miniaturized in its width and so the shape of the thin film magnetic head must be formed with high accuracy.
Now, the structure of a thin film magnetic head will be explained. FIG. 1 is a perspective view of a thin film magnetic head with a part thereof cut away. Formed on a substrate 11 is a lower magnetic film 12 which serves as a magnetic core which constitutes a magnetic circuit together with an upper magnetic film 13 which will be formed later. Formed at the tip portion 17 is a magnetic gap which is made by inserting a non-magnetic film 14 between the magnetic films 12 and 13 (This gap is used to carry out the read or write for a recording medium). On the other hand, formed at the central portion of the magnetic core is conductor coils 16 which intersect the magnetic circuit. The conductor coils are insulated from the magnetic films 12 and 13 by an insulating film 15.
The track width which determines the recording density of the above thin film magnetic head is generally decided by the width d of the upper magnetic film 13 at the head tip portion 17. Then, in order to realize the track width with high accuracy, it is necessary to form the upper magnetic film 13 with accuracy as high as .+-.0.5 .mu.m or less at the lower part of a stepped portion of the insulating film 15 having a height of about 10 .mu.m. To this end, in many cases, the dry etching technique, which is easy to control etching amount and can provide the high accuracy, and particularly the ion milling technique using accelerated ions are used. Incidentally, the thickness of the magnetic films 12, 13 and the magnetic gap 14, which are different depending on the type of magnetic head, are usually about 1 to 4 .mu.m, and about 0.2 to 1 .mu.m, respectively. The track width is usually 2 to 30 .mu.m.
One example of the methods of forming a thin film pattern at a stepped portion as mentioned above is disclosed in Japanese patent unexamined publication JP-A-60-37130 which proposes a method of patterning a thin film at a stepped portion through the ion milling technique using photosensitive resin as a mask. This method will be explained with reference to FIGS. 2A to 2C. First, as shown in FIG. 2A, a step 22 is formed on a base 21 and a thin film 23 to be patterned is formed on the surface of the resultant structure. As shown in FIG. 2B, a photosensitive resin film 24 is applied on the thin film 23 and is patterned. Finally, as shown in FIG. 2C, the thin film 23 is etched through the ion milling technique using the photosensitive resin film 24 as a mask to provide an objective pattern shape. In the case of the thin film magnetic head, the base 21 corresponds to the layers including the substrates 11, magnetic film 12 and gap 14 as shown in FIG. 1 and the stepped portion 22 corresponds to the resin insulating film 15 of FIG. 1.
The above reference JP-A-60-37130 also proposes a method of implementing patterning with higher accuracy than in the method mentioned above. This will be explained with reference to FIGS. 3A to 3D. First, as shown in FIG. 3A, the structure as shown in FIG. 2A is formed and thereafter an alumina film 35 is formed thereon. As shown in FIG. 3B, a photosensitive resin film 34 is formed on the surface of the resultant structure and, as shown in FIG. 3C, the alumina film 35 is patterned through the reactive ion milling technique using gas containing fluorine atoms, for example CF.sub.4, CHF.sub.3 or C.sub.2 F.sub.6. Finally, as shown in FIG. 3D, the photosensitive resin film 34 is removed and thereafter a thin film 33 is patterned through the ion milling technique using the alumina film 35 as a mask, thus providing an objective pattern shape. In FIGS. 3A to 3D, 31 is a base and 32 is a stepped portion.
The prior art shown in FIGS. 2A to 2C has the following disadvantage. When the photosensitive resin film 24 used as a mask is applied with its required minimum thickness of about 3 .mu.m at the upper part of the stepped portion 22, it results in its thickness as high as about 10 to 20 .mu.m at the lower part of the stepped portion 22. Thus, the etched material is re-deposited on the sidewall of the resin pattern as formed thereby producing a re-deposited layer 25 as shown in FIG. 2D so that the dimension accuracy of the thin film 23 at the lower part of the stepped portion will be reduced and a desired pattern configuration can not be obtained. FIG. 2D shows a section when the thin film pattern at the step lower part obtained in FIG. 2C is viewed from the left side of FIG. 2C.
The prior art shown in FIGS. 3A to 3D has the following disadvantage. The alumina film 35 used as a mask can be made as thin as about 2 .mu.m so that the above re-deposited layer will not be produced. However, this method requires two steps of transferring a pattern of the photosensitive resin film 34 on the alumina film 35 and again transferring it on the thin film 33 to provide a pattern thereof. This will deteriorate the dimension accuracy of the objective pattern. Namely, the prior art shown in FIGS. 3A to 3D has a limit of .+-.0.8 .mu.m for the pattern dimension accuracy so that it is difficult to provide a pattern with the accuracy of .+-.0.5 .mu.m or less.